Line circuits for key systems are well-known. The progression and advance may be seen from the essentially electromagnetic circuits shown in U.S. Pat. No. 3,239,610 issued Mar. 8, 1966 to Morse and the later more transistorized versions shown in U.S. Pat. No. 3,436,488 issued Apr. 1, 1969 to Barbato et al. A later version is shown by the more recent U.S. Pat. No. 3,676,608 issued July 11, 1971 to Goldthorp et al.